Constant velocity universal joints (CV joints) are sometimes used in a drive shaft of a vehicle. CV joints connect two shafts on a drive side and a driven side such that a rotational force can be transmitted at a constant velocity even when there is an angle between the two shafts. A CV joint including a leg shaft and a roller (for example, a tripod constant velocity universal joint) is known. In the case of the tripod constant velocity universal joint, an inner joint member is connected to one shaft, an outer joint member is connected to the other shaft, and a roller fitted to the leg shaft is housed in the guide groove of the outer joint member, whereby the two shafts are connected to each other and torque is transmitted. The inner joint member includes three leg shafts that protrude in a radial direction. The outer joint member is a hollow cylinder that includes three guide grooves that extend in an axial direction of the outer joint member.
In a known tripod type CV joint (such as that shown in FIG. 10, for example), a roller 6 includes an inner roller 6b and an outer roller 6a that can be moved in the axial direction with respect to each other such that the roller 6 can be moved in parallel along a guide groove 2a. A convex sphere is formed in a tip portion of a leg shaft 5a, and a concave sphere is formed in an inner peripheral surface of inner roller 6b such that leg shaft 5a and the inner roller 6b can be oscillated with respect to each other (for example, refer to Japanese Patent Laid-Open Publication No. 2002-147482, the entirety of which is incorporated herein by reference). With this configuration, when a CV joint 1 is rotated at an angle (the joint angle), inner roller 6b fitted to leg shaft 5a is moved in the axial direction with respect to outer roller 6a. However, outer roller 6a is moved only in parallel along guide groove 2a. Therefore, less friction occurs as compared to when the entire roller 6 is displaced in the axial direction. Thus, it is possible to suppress a thrust force of outer joint member 2 in the axial direction that is generated due to the friction. In turn, it is possible to suppress vibration caused by this thrust force.
In a CV joint having the aforementioned structure, the outer roller may make angular contact with the guide groove of the outer joint member in order to make the posture of the outer roller stable. FIG. 11 shows a case where the outer roller 6a makes angular contact with the guide groove 2a of the outer joint member 2. The outer roller 6a makes contact with the guide groove 2a at contact points A and B. Points A and B are symmetrical with respect to a plane that passes through the center of outer roller 6a in the axial direction and that is perpendicular to the axis.
However, when the outer roller makes angular contact with the groove of the outer joint member, since a contact point between the leg shaft and the inner roller is moved due to rotation of the joint, a thrust force is generated in the axial direction of the outer joint member (the Z-axis direction). This thrust force causes vibration in the CV joint member, as described in detail below.
Referring to FIG. 11, when CV joint 1 is rotated by a joint angle, leg shaft 5a and inner roller 6b are moved in the axial direction of inner roller 6b (the Y-axis direction), and friction occurs between inner roller 6b and a needle bearing 7. Therefore, the contact point between leg shaft 5a and inner roller 6b is moved along the inner sphere of inner roller 6b as shown by arrow D so that force balancing with the frictional force is generated at the contact point.
When the contact point between leg shaft 5a and inner roller 6b is moved as shown by arrow D and as described above, moment Mz around the Z-axis is generated between outer roller 6a and needle bearing 7. In order to balance with moment Mz, a contact load is generated, for example, at a point K on a rear surface side which is opposed to a side where a load is applied. When roller unit 6 is moved in the Z-axis direction while the contact load is applied, a frictional force Rk is generated at point K. Further, moment My around the Y-axis is generated due to frictional force Rk. Therefore, in order to balance moment My generated due to the frictional force Rk, frictional forces Ra and Rb are generated also at contact points A and B (respectively) between outer roller 6a and outer joint member 2 on the side where the load is applied. FIG. 12 is a diagram showing the directions of frictional forces Ra and Rb. FIG. 12 a schematic arrow cross-sectional view taken along line XII-XII in FIG. 11. As shown in FIG. 12, the frictional forces Ra and Rb, which are generated at the contact points A and B in order to make the moment My zero, are applied in the same direction as the direction in which the frictional force Rk is applied. Therefore, the thrust force is a resultant force of the three frictional forces Rk, Ra, and Rb, as expressed in Equation 1. Also, frictional forces Ra and Rb are obtained according to Equation 2, which indicates the balance between frictional forces Ra and Rb and moment My. Thus, the large thrust force in the Z-axis direction is generated when the contact point between leg shaft 5a and inner roller 6b is moved.Thrust force=−(Rk+Ra+Rb)  (Equation 1)My=Rk×d1−(Ra+Rb)×d2=0.
In Equation 2, d1 indicates a length in an X-axis direction from an axis of the inner roller to point K, and d2 indicates a length in the X-axis direction from the axis of the inner roller to point A (or point B).